1.2 - Diffusion & Particle Motion

What diffusion is

• In a liquid or a gas, particles are in constant, random motion.
• Diffusion is the spreading out of particles from a region of higher concentration to a region of lower concentration, until they're evenly spread.
• The greater the difference in concentration, the faster diffusion happens.
• Once particles are evenly spread, they continue to move, but there's no further overall change.

Diffusion happens in liquids and gases only

• It happens in liquids and gases because the particles are free to move past one another.
• It does not happen in a solid - each particle is held in a fixed position and can only vibrate on the spot, so it can't move from place to place.

How fast a gas diffuses depends on particle mass

• At the same temperature, a gas with lighter particles diffuses faster than one with heavier particles.
• If two gases start diffusing at the same time, the one with lighter particles travels further in the same time.

Evidence for diffusion

Diffusion in a liquid - Potassium manganate(VII)

1. Drop a crystal of potassium manganate(VII) into a beaker of water.
2. Left undisturbed, the purple colour spreads slowly until the whole beaker is a uniform pale purple, as the potassium manganate(VII) particles diffuse among the water particles.
3. Adding more water dilutes the solution: the particles spread further apart throughout the larger volume, so the colour looks paler.

Diffusion between gases - Bromine and air

1. Set up two gas jars mouth to mouth, one filled with brown bromine gas and the other with air, separated by a glass plate.
2. When the plate is removed, the brown gas spreads into the jar of air until it's evenly spread through both jars.

Different gases diffuse at different speeds - Ammonia and HCl

1. Push a cotton wool plug soaked in aqueous ammonia into one end of a glass tube and a plug soaked in hydrochloric acid into the other, then seal both ends.
2. Ammonia gas and hydrogen chloride gas diffuse along the tube towards each other.
3. Where they meet, they react to form a white ring of solid ammonium chloride.
4. The ring forms nearer the hydrochloric acid end because ammonia particles are lighter, so they diffuse faster and travel further in the same time.